Stencil screens, commonly known as silk screens, are usually prepared by coating a screen material with a photosensitized emulsion. Selective portions of the screen are exposed to light and the screen is developed to harden the emulsion in desired areas. The screens produced by these methods are generally satisfactory for most purposes, but are somewhat deficient when used for printed circuits where a relatively large thickness of material, such as solder paste, must be deposited. They are also unsatisfactory where controlled and/or uniform thickness is required.
The prior art has pursued this problem of producing relatively thick printed circuits without substantial success. According to the prior art, a stencil is prepared by coating a screen with a photoresist emulsion, and then recoating the screen several times. The deficiency of this method is that the emulsion has a tendency to run and produce high and low spots, rendering the stencil unsatisfactory for precise printed patterns. In some instances, the screen commonly of stainless steel, is placed on the emulsion while wet. Then, uniform pressure is applied to the assembly, and the emulsion is allowed to dry under pressure. A positive transparency is then applied to the screen. The stencil is then exposed to light. The exposed emulsion hardens and the remaining emulsion is dissolved and washed away. The resulting stencil is characterized by identical hardened emulsion patterns on both sides of the screen. This process is also very time consuming, not only in the use of the many steps, but the time required to dry the photoresist.
Similar prior art systems utilize pliable photoresist film that is exposed while wet and applied to the opposite sides of the screen. The photoresist is then dried and developed. In either of the previously mentioned methods, the stencil is placed in a chase and placed over a part such as a printed circuit board. The desired material, such as a solder paste, is deposited on the part by squeegeeing through the openings defined by the hardened emulsion on the screen.
The prior art processes as described above and other conventional processes are, to a limited extent, capable of producing a stencil of adequate thickness for most applications. The processes, however, are deficient in that they require a multiplicity of delicate steps. It is necessary to subject the wet emulsion to excessive handling. Extreme care is often required to prevent injuring the integrity of the emulsion. Furthermore, it is difficult to avoid improper handling of the emulsion since the stencil must be processed in various pieces of equipment while still wet. There are several other alternative steps and other methods utilized to facilitate the preparation of a stencil. However, none of these other processes are effective to substantially cure the deficiencies of present stencil production.
The prior art is exemplified by the following U.S. Patents:
U.S. Pat. No. 2,969,732, issued January of 1961 to Kendall, discloses the above described method of making a stencil by applying coatings of wet photo sensitive resins to a supporting base and thereafter using the coated supporting base to form a stencil. This disclosed process has the deficiencies as pointed out above, including the resultant inaccuracy of the stencil as well as the time consuming process of making the stencil.
U.S. Pat. No. 2,949,848, issued August of 1960 to Mott, discloses a stencil making method wherein duplicating stencils are prepared xerographically. In the process a screen is attached to a tacky binder material in the process of making the stencil. The layer of material is put in a tacky state by the application of heat or a solvent and is attached by placing the screen over the tacky material and applying a pressure, forcing it into the tacky layer.
U.S. Pat. No. 3,100,702, issued August of 1963 to Rauner et al, discloses a dry processed photothermographic printing plate and process. This patent discloses the transfer of softened matrix to a support, wherein light sensitive resin of a matrix sheet is hardened in exposed areas. The matrix material is heated to soften it, and thereafter is transferred to a sheet or duplicating master sheet. This patent, of course, does not relate to the process of stencil screen making, but to the process of printing plate making.
U.S. Pat. No. 3,507,654, issued April of 1970 to Wrench, is directed to a stencil screen and method. This patent discloses a plurality of steps in a method of applying a photo sensitive emulsion over a coating and then disposing a screen on the layer of wet emulsion, causing the emulsion to permeate the screen. The emulsion is then exposed to a light through a positive, and the unexposed areas washed out. This again is the application of a wet emulsion photo sensitive material to a screen support somewhat similar to the method disclosed in the above described Kendall patent.
U.S. Pat. No. 3,532,052, issued October of 1970 to Erickson; and U.S. Pat. No. 3,676,128, issued July of 1972 to Leopold et al; both disclose somewhat similar techniques of applying dry film photoresist to a screen. However, the dry film photoresist in both disclosed processes are not applied in the dry film state. In both instances, the film is made to adhere to the support screen by softening the film by means of a solvent. In other words the formerly dry film photoresist is turned to a wet film before applying to a screen. In Erickson, for example, Column 3 lines 6 through 19, the technique is described as including "a liquid emulsion for securing this layer to the contact side of a screen, the liquid emulsion being applied to the screen from the top side thereof through and substantially filling the mesh of the screen and a liquid emulsion having characteristics of joining and becoming homogenious with a prepared contact layer of emulsion and thereby forming a complete colloid which has uniform contact layer and a firm mounting on a screen material". These patents, for example, make it abundantly clear that they did not consider it possible to apply a dry film photoresist to a support screen with the photoresist essentially in its dry state. The Leopold patent, similarly describes a process, in Column 2 for example, lines 15 through 22, wherein a complex which is made to adhere to a gauze from the inner side of the frame by wetting it with a solvent. In Column 1, lines 68 through 70, this patent describes a complex film as adapted to be supplied to the operator in rolls or, alternately, it is pre-cut to size. This apparently refers to material, such as that known as Riston material.
U.S. Pat. No. 3,469,982, issued September of 1969 to Celeste, discloses a process for making photoresist. A photoresist solution is applied to the surface of a thin transparent film as a support backing. There is no disclosure or suggestion therein of applying it or constructing a stencil therefrom.
U.S. Pat. No. 3,853,561, issued December of 1974 to Reichel et al, discloses a process for the preparation of a screen printing stencil using intermediate support for light sensitive layers. The specific disclosure therein is of the use of a specific composition of photoresist materials applied wet to a sheet-like intermediate support. Again a wet coating solution is applied to a screen member as in the previously discussed prior art.
German Pat. No. 2,050,285, issued May of 1972 to Siemens, discloses a method of producing a screen printing plate by laminating a photo sensitive foil to a polished support plate and forming the desired image on the polished plate for the printing characters. This patent simply discloses a technique for making a screen from a flat metal support sheet. It does not suggest the use of dry film photoresist in the construction of a screen stencil or the application of dry film photoresist to a open mesh screen.
The aforementioned patents comprise a fairly comprehensive coverage of the prior art techniques relating to art to which the present invention relates. The deficiencies of these methods as disclosed in the prior art become apparent in comparison to the applicant's invention. In summary, prior art has simply failed to solve the problem for which the applicants invention was devised.
Accordingly, there is a recognized need for a stencil production method that is characterized by simplicity and efficiency and requires minimum handling of wet photoresist.